Axial fan

ABSTRACT

An axial fan includes a rotor portion including a shaft arranged to extend along a central axis extending in a vertical direction, a stator portion arranged radially opposite to the rotor portion, and an impeller hub fixed to the rotor portion, and arranged to be capable of rotating integrally with the rotor portion. The impeller hub includes a hub top plate portion arranged to extend perpendicularly to an axial direction; a hub tubular portion being tubular, and arranged to extend axially downward from an outer edge of the hub top plate portion; a plurality of blades arranged in a circumferential direction on an outer surface of the hub tubular portion; a plurality of wall portions arranged in the circumferential direction radially inside of the hub tubular portion, with a gap between every adjacent ones of the wall portions; and joining portions each of which is arranged to join a corresponding one of the wall portions to the hub tubular portion. The rotor portion includes a rotor tubular portion being tubular and arranged to extend in the axial direction. An outer surface of the rotor tubular portion is arranged to be in contact with an inner surface of at least one of the wall portions. Each of the at least one wall portion arranged to be in contact with the outer surface of the rotor tubular portion is joined to the hub tubular portion through two or more of the joining portions arranged in the circumferential direction.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to Japanese PatentApplication No. 2017-187687 filed on Sep. 28, 2017. The entire contentsof this application are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present disclosure relates to an axial fan.

2. Description of the Related Art

In a known fan, an impeller is attached to a rotor yoke. The impellerincludes an impeller cup including a tubular lateral wall portion in theshape of a truncated cone, and a plurality of blades arranged in anannular shape and arranged to project from an outer circumferentialsurface of the lateral wall portion of the impeller cup. In addition,inside of the lateral wall portion of the impeller cup, an annularmember is defined concentrically and integrally with the lateral wallportion of the impeller cup. A plurality of support members are arrangedin a circumferential direction between an inner circumferential surfaceof the lateral wall portion of the impeller cup and an outercircumferential surface of the annular member. Further, a plurality ofribs are arranged to project radially inward from the annular member.When the rotor yoke is press fitted to the impeller cup, a radiallyoutward stress is applied to each rib. A radially outward press-fittingstress thus acting on each rib at the time of the press fitting isabsorbed by the annular member as a circumferential force, resulting ina reduced load on the impeller cup.

SUMMARY OF THE INVENTION

In recent years, there has been a demand for a high air volume fan motorcapable of continuously operating under a high temperature condition.The flexural modulus of elasticity tends to decrease under the hightemperature condition, and it is therefore desirable to choose amaterial having a high flexural modulus of elasticity to prevent adeformation of an impeller blade portion. In addition, a demand forhigher air volumes requires the fan motor to rotate at a high speed, andtherefore, an impeller cup and a rotor yoke need to be securely fixed toeach other to maintain balance of an impeller even when the fan motor isrotating at a high speed under the high temperature condition.Meanwhile, in the case where the impeller cup is made of a materialhaving a high flexural modulus of elasticity, a thermal stress caused bya difference in thermal expansion between the impeller cup and the rotoryoke tends to become significantly high when a significant decrease inambient temperature occurs. Moreover, when a significant increase in theambient temperature occurs, a difference in the thermal expansionbetween the impeller cup and the rotor yoke may cause a reduction instrength with which the impeller cup and the rotor yoke are fixed toeach other. Accordingly, there is a demand for a fan motor capable ofcontinuously operating with stability with the ability to prevent asignificantly high thermal stress from acting on an impeller or a rotoryoke, and to maintain secure fixing of the impeller and the rotor yoketo each other, even when a change in ambient temperature occurs.

An axial fan according to a preferred embodiment of the presentdisclosure includes a rotor portion including a shaft arranged to extendalong a central axis extending in a vertical direction; a stator portionarranged radially opposite to the rotor portion; and an impeller hubfixed to the rotor portion, and arranged to be capable of rotatingintegrally with the rotor portion. The impeller hub includes a hub topplate portion arranged to extend perpendicularly to an axial direction;a hub tubular portion being tubular, and arranged to extend axiallydownward from an outer edge of the hub top plate portion; a plurality ofblades arranged in a circumferential direction on an outer surface ofthe hub tubular portion; a plurality of wall portions arranged in thecircumferential direction radially inside of the hub tubular portion,with a gap between every adjacent ones of the wall portions; and aplurality of joining portions each of which is arranged to join acorresponding one of the wall portions to the hub tubular portion. Therotor portion includes a rotor tubular portion being tubular andarranged to extend in the axial direction. An outer surface of the rotortubular portion is arranged to be in contact with an inner surface of atleast one of the wall portions. Each of the at least one wall portionarranged to be in contact with the outer surface of the rotor tubularportion is joined to the hub tubular portion through two or more of thejoining portions arranged in the circumferential direction.

The above and other elements, features, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of the preferred embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating an axial fan according to apreferred embodiment of the present disclosure.

FIG. 2 is a plan view of the axial fan illustrated in FIG. 1.

FIG. 3 is a vertical sectional view of the axial fan illustrated in FIG.1.

FIG. 4 is a perspective view of a housing according to a preferredembodiment of the present disclosure.

FIG. 5 is a perspective view of a stator portion according to apreferred embodiment of the present disclosure.

FIG. 6 is a perspective view of a rotor yoke according to a preferredembodiment of the present disclosure.

FIG. 7 is a perspective view of an impeller according to a preferredembodiment of the present disclosure.

FIG. 8 is a perspective view of the impeller illustrated in FIG. 7 asviewed from below.

FIG. 9 is a plan view of the impeller illustrated in FIG. 7.

FIG. 10 is a bottom view of the impeller illustrated in FIG. 7.

FIG. 11 is a plan view illustrating a circumferential development of ablade attached to an impeller hub according to a preferred embodiment ofthe present disclosure.

FIG. 12 is a diagram depicting circumferential developed bladesaccording to a preferred embodiment of the present disclosuresuperimposed on each other, the circumferential developed blades beingcircumferential developments of circumferential sections of the bladetaken at different radial positions.

FIG. 13 is a schematic bottom view illustrating an arrangement of aninner fixing portion according to a preferred embodiment of the presentdisclosure.

FIG. 14 is a vertical sectional view of an impeller used in an axial fanaccording to another preferred embodiment of the present disclosure.

FIG. 15 is a schematic bottom view of one of first wall portionsincluded in the impeller illustrated in FIG. 14.

FIG. 16 is a bottom view of an impeller used in an axial fan accordingto yet another preferred embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present disclosure will bedescribed in detail with reference to the accompanying drawings. It isassumed herein that, regarding an axial fan A, a direction parallel to acentral axis C1 of the axial fan A is referred to by the term “axialdirection”, “axial”, or “axially”, that directions perpendicular to thecentral axis C1 of the axial fan A are each referred to by the term“radial direction”, “radial”, or “radially”, and that a direction alonga circular arc centered on the central axis C1 of the axial fan A isreferred to by the term “circumferential direction”, “circumferential”,or “circumferentially”. It is also assumed herein that, regarding theaxial fan A, an axial direction is a vertical direction, and that a sideon which an air inlet 16 of a housing 10 is arranged with respect to animpeller 40 is defined as an upper side. The shape of each member orportion and relative positions of different members or portions will bedescribed based on the above assumptions. It should be noted, however,that the above definition of the vertical direction and the upper andlower sides is made simply for the sake of convenience in description,and is not meant to restrict relative positions or directions ofdifferent members or portions of the axial fan A when in use. It is alsoassumed herein that an upstream side and a downstream side are definedwith respect to a direction in which an air flow caused by rotation ofthe impeller 40 passes.

FIG. 1 is a perspective view illustrating an axial fan A according to afirst preferred embodiment of the present disclosure. FIG. 2 is a planview of the axial fan A illustrated in FIG. 1. FIG. 3 is a verticalsectional view of the axial fan A illustrated in FIG. 1.

Referring to FIGS. 1 to 3, the axial fan A according to the presentpreferred embodiment includes a housing 10, a stator portion 20, a rotorportion 30, and an impeller 40. The stator portion 20 is fixed to thehousing 10. The rotor portion 30 is arranged to be capable of rotatingwith respect to the stator portion 20, and includes a portion arrangedradially outside of the stator portion 20 with a gap therebetween. Theimpeller 40 is attached to the rotor portion 30.

The housing 10 will now be described below with additional reference toFIG. 4. FIG. 4 is a perspective view of the housing 10. In theperspective view illustrated in FIG. 4, a shaft 31, which will bedescribed below, of the rotor portion 30 is also depicted.

The housing 10 includes an air channel wall portion 11, a base portion12, stationary vanes 13, a bearing holding tube portion 14, and flangeportions 15. The air channel wall portion 11 includes a cylindricalinner surface arranged to extend along the central axis C1. The impeller40 is arranged to rotate inside of the air channel wall portion 11. Theair channel wall portion 11 is a guide arranged to guide an air flowcaused by the rotation of the impeller 40 along the central axis C1. Anair inlet 16 is defined at an axially upper end of the air channel wallportion 11, while an air outlet 17 is defined at an axially lower end ofthe air channel wall portion 11. That is, the rotation of the impeller40 causes air to be sucked through the air inlet 16, and causes the airflow, being accelerated or pressurized by the impeller 40, to bedischarged through the air outlet 17.

The flange portions 15 are arranged to extend radially outward from eachof both axial end portions of the air channel wall portion 11. Eachflange portion 15 includes a fitting hole 151 arranged to passtherethrough in the axial direction. The fitting hole 151 is used whenthe axial fan A is attached to a device. Specifically, a fitting screw,a boss, or the like provided in the device is inserted into the fittinghole 151 to fix the flange portion 15 to the device, so that the axialfan A is fixed to the device. The flange portions 15 are arranged in theform of a square as illustrated in FIGS. 1, 2, and 4, but mayalternatively be arranged in the form of a circle, a rectangle, oranother polygon, such as, for example, a hexagon. The form of the flangeportions 15 may be determined in accordance with the form of aportion(s) of the device to which the axial fan A is attached.

The base portion 12 is arranged to hold the stator portion 20. The baseportion 12 includes, in a center thereof, a base through hole 120 (seeFIG. 3) arranged to pass therethrough in the axial direction, and alsoincludes a tubular tube holding portion 121 arranged to project axiallyupward above a peripheral portion of the base through hole 120.

The base portion 12 is arranged at the axially lower end of the airchannel wall portion 11, i.e., at an end of the air channel wall portion11 on the downstream side with respect to the air flow. The base portion12 is arranged radially inside of the air channel wall portion 11. Theair channel wall portion 11 and the base portion 12 are radially spacedapart from each other. The stationary vanes 13 are arranged in acircumferential direction in a gap between the air channel wall portion11 and the base portion 12. Each stationary vane 13 is joined to boththe air channel wall portion 11 and the base portion 12. In other words,the base portion 12 is held by the air channel wall portion 11 throughthe stationary vanes 13. The stationary vanes 13 are arranged to controlair flows caused by the rotation of the impeller 40 so that the airflows will be axially symmetric with respect to the central axis C1.Accordingly, the stationary vanes are arranged at regular intervals inthe circumferential direction. The base portion 12 defines a portion ofthe housing 10, but the base portion 12 may alternatively be defined bya member separate from the housing 10.

The bearing holding tube portion 14 is cylindrical, and the statorportion 20 is fixed to an outer circumferential surface of the bearingholding tube portion 14. The bearing holding tube portion 14 is fixed tothe tube holding portion 121 of the base portion 12 along the centralaxis C1. The bearing holding tube portion 14 is arranged to hold a firstbearing 141 and a second bearing 142 with inner circumferential surfacesof an axially upper end portion and an axially lower end portion,respectively, thereof. As illustrated in FIG. 3, the first bearing 141is arranged on the axially upper end portion, while the second bearing142 is arranged on the axially lower end portion. The first and secondbearings 141 and 142 are arranged to rotatably support the shaft 31,which will be described below, of the rotor portion 30.

The bearing holding tube portion 14 is fixed to the tube holding portion121 of the base portion 12 such that the bearing holding tube portion 14is coaxial with the central axis C1. Accordingly, a center of the statorportion 20, which is fixed to the outer circumferential surface of thebearing holding tube portion 14, coincides with the central axis C1. Inaddition, a center of the shaft 31, which is rotatably supported by thebearing holding tube portion 14 through the first and second bearings141 and 142, coincides with the central axis C1. That is, both thecenter of the stator portion 20 and a center of the rotor portioncoincide with the central axis C1. Thus, a radially outer surface ofeach of tooth portions 212, which will be described below, of the statorportion 20 is arranged radially opposite to an inner circumferentialsurface of a rotor magnet 34, which will be described below, of therotor portion 30 with a predetermined distance therebetween. That is,the stator portion 20 is arranged radially opposite to the rotor portion30.

Each of the first and second bearings 141 and 142 is a ball bearing. Theshaft 31 is fixed to an inner race of each of the first and secondbearings 141 and 142. The shaft 31 is fixed to the inner race of each ofthe first and second bearings 141 and 142 through, for example,insertion and adhesion, press fitting, or the like, or by other fixingmethods. Note that each of the first and second bearings 141 and 142 isnot limited to the ball bearing.

The stator portion 20 will now be described in detail below withadditional reference to FIG. 5. FIG. 5 is a perspective view of thestator portion 20. Referring to FIGS. 3, 5, and so on, the statorportion 20 includes a stator core 21, an insulator 22, and coils 23. Thestator core 21 has electrical conductivity. The stator core 21 includesan annular core back portion 211 and the tooth portions 212. The coreback portion 211 is annular, and is arranged to extend in the axialdirection. Each tooth portion 212 is arranged to project radiallyoutward from an outer circumferential surface of the core back portion211. The number of tooth portions 212 included in the stator core 21 istwo or more. The tooth portions 212 are arranged at regular intervals inthe circumferential direction.

The stator core 21 may be defined by laminated electromagnetic steelsheets, or may alternatively be defined in one piece by sintering ofpowder, casting, or the like. The stator core 21 may be made up of coresegments each of which includes one of the tooth portions 212, or mayalternatively be defined by winding a strip-shaped member. The statorcore 21 has, in a radial center thereof, a through hole arranged to passtherethrough in the axial direction.

The insulator 22 is a resin casting. The insulator 22 is arranged tocover at least each tooth portion 212 of the stator core 21 in itsentirety. A conducting wire is wound around each tooth portion 212covered with the insulator 22 to define the corresponding coil 23. Theinsulator 22 provides isolation between the stator core 21 and each coil23. Although the insulator 22 is a resin casting in the presentpreferred embodiment, this is not essential to the present invention.Other types of insulator that are able to provide isolation between thestator core 21 and each coil 23 can be widely adopted as the insulator22.

The coil 23 is arranged around each of the tooth portions 212 of thestator core 21. The coils 23 included in the stator portion 20 can bedivided into three groups (hereinafter referred to as three phases)which differ in timing of supply of an electric current. The threephases are defined as a U phase, a V phase, and a W phase, respectively.That is, the stator portion 20 includes U-phase coils, V-phase coils,and W-phase coils, all of which are equal in number. Hereinafter, thecoils of the three phases will be simply referred to collectively as thecoils 23.

The stator portion 20 is fixed to the bearing holding tube portion 14with a wall surface of the through hole of the stator core 21 being incontact with the outer circumferential surface of the bearing holdingtube portion 14. The stator core 21 and the bearing holding tube portion14 may be fixed to each other through press fitting, adhesion, or thelike, or by other fixing methods. Various methods by which the statorcore 21 can be securely fixed to the bearing holding tube portion 14 canbe widely adopted.

With the stator core 21 being fixed to the bearing holding tube portion14, the stator portion 20 is fixed to the base portion 12, i.e., insideof the air channel wall portion 11 of the housing 10. As a result, thetooth portions 212 are arranged at regular intervals around the centralaxis C1.

Referring to FIG. 3, the rotor portion 30 includes the shaft 31, a rotoryoke 33, and the rotor magnet 34. The shaft 31 is columnar. The shaft 31is arranged to extend in the axial direction along the central axis C1.The rotor yoke 33 is made of a metal. That is, the rotor portion 30includes the shaft 31, which is arranged to extend along the centralaxis C1 extending in the vertical direction.

The rotor yoke 33 will now be described in detail below with additionalreference to FIG. 6. FIG. 6 is a perspective view of the rotor yoke 33.Referring to FIG. 6, the rotor yoke 33 includes a rotor top plateportion 331 and a rotor tubular portion 332. The rotor top plate portion331 is arranged to extend radially, and is in the shape of a disk whenviewed in the axial direction. The rotor top plate portion 331 includes,in a center thereof, a central through hole 333 arranged to passtherethrough in the axial direction. The rotor top plate portion 331includes a plurality (four in the present preferred embodiment) ofpositioning holes 334 each of which is arranged to pass therethrough inthe axial direction. First bosses 413, which will be described below, ofthe impeller 40 are inserted into the positioning holes 334.

The rotor tubular portion 332 is tubular, and is arranged to extendaxially downward from a radially outer edge of the rotor top plateportion 331. The rotor tubular portion 332 is fixed to an inner fixingportion 43, which will be described below, of the impeller 40 throughpress fitting. A coupling portion 32 is inserted into the centralthrough hole 333. That is, the rotor portion 30 includes the rotortubular portion 332 being tubular and arranged to extend in the axialdirection.

The coupling portion 32 is arranged to couple and fix the rotor topplate portion 331 and the shaft 31 to each other. The coupling portion32 includes a coupling hole 321, a yoke fixing portion 322, and acoupling tube portion 323. The coupling tube portion 323 is tubular,extending in the axial direction. The yoke fixing portion 322 isarranged at an axially lower end of the coupling tube portion 323. Thecoupling hole 321 is arranged to pass through the coupling tube portion323 in the axial direction.

An axially upper end portion of the shaft 31 is inserted into thecoupling hole 321. The axially upper end portion of the shaft 31 ispress fitted into the coupling hole 321 to be fixed to the couplingportion 32. The yoke fixing portion 322 is inserted into the centralthrough hole 333 of the rotor yoke 33. The yoke fixing portion 322includes a cylindrical outer surface arranged to be in contact with andfixed to a wall surface of the central through hole 333. The couplingtube portion 323 is inserted into an axial through hole 414, which willbe described below, of the impeller 40, and is fixed in the axialthrough hole 414. The coupling tube portion 323 may be fixed in theaxial through hole 414 through, for example, adhesion, welding, or thelike, or by other fixing methods.

The coupling portion 32 is arranged to fix the shaft 31 and the impeller40 to each other, and fix the shaft 31 and the rotor yoke 33 to eachother. In other words, each of the impeller 40 and the rotor yoke 33 isfixed to the shaft 31 through the coupling portion 32.

The rotor magnet 34 is tubular, and includes north and south polesarranged to alternate with each other in the circumferential direction.The rotor magnet 34 is fixed with an outer circumferential surfacethereof being in contact with an inner circumferential surface of therotor yoke 33. The rotor magnet 34 may be molded in one piece of a resincontaining magnetic powder, or may alternatively be defined by aplurality of magnets arranged in the circumferential direction and fixedto one another through a resin or the like. The rotor magnet 34 may befixed to the rotor yoke 33 through press fitting, adhesion, or the like,or by other fixing methods. Various methods by which the rotor magnet 34can be securely fixed to the rotor yoke 33 can be widely adopted.

The shaft 31 is rotatably attached to the bearing holding tube portion14 through the first and second bearings 141 and 142 held by the bearingholding tube portion 14. Then, the rotor yoke 33 with the rotor magnet34 fixed thereto is fixed to the shaft 31 through the coupling portion32. At this time, the radially inner circumferential surface of therotor magnet 34 is arranged radially opposite to the radially outersurface of each of the tooth portions 212 of the stator portion 20,which is fixed to the bearing holding tube portion 14, with a gaptherebetween. The base portion 12, the bearing holding tube portion 14,the stator portion 20, and the rotor portion 30 together define abrushless DC motor of a so-called outer-rotor type, in which the rotormagnet 34 of the rotor portion 30 is arranged radially outside of thestator portion 20. Although the base portion 12 defines a portion of thehousing 10 in the present preferred embodiment, the base portion 12 mayalternatively be defined by a member separate from the housing 10. Inthis alternative case, the motor may be assembled separately, and beattached to the housing 10.

Magnetic flux generated as a result of electric currents being passedthrough the coils 23 of the stator portion 20 causes an attractive forceor a repulsive force to be applied to the rotor magnet 34. Theattractive force or the repulsive force applied to the rotor magnet 34causes the rotor portion 30 to rotate about the central axis C1 withrespect to the stator portion 20. Rotation of the rotor portion 30causes the impeller 40 fixed to the rotor portion 30 to rotate about thecentral axis C1.

The impeller 40 will now be described in detail below with additionalreference to FIGS. 7, 8, 9, and 10. FIG. 7 is a perspective view of theimpeller 40. FIG. 8 is a perspective view of the impeller 40 illustratedin FIG. 7 as viewed from below. FIG. 9 is a plan view of the impeller 40illustrated in FIG. 7. FIG. 10 is a bottom view of the impeller 40illustrated in FIG. 7.

Referring to FIGS. 7 to 10, the impeller 40 includes an impeller hub 41,a plurality of blades 42, and the inner fixing portion 43. The impeller40 is defined by a resin injection molding process.

Referring to FIGS. 3, 7, 8, and so on, the impeller hub 41 includes ahub top plate portion 411 and a hub tubular portion 412. The hub topplate portion 411 is in the shape of a disk, extending radially. The hubtubular portion 412 is tubular, and is arranged to extend axiallydownward from a radially outer edge of the hub top plate portion 411.The hub top plate portion 411 is provided with the first bosses 413, theaxial through hole 414, and second bosses 415. The axial through hole414 is a through hole arranged to pass through the hub top plate portion411 in the axial direction, and arranged in a radial center of the hubtop plate portion 411. The coupling tube portion 323 of the couplingportion 32 is inserted into and fixed in the axial through hole 414. Inother words, the shaft 31 is fixed in the axial through hole 414 throughthe coupling tube portion 323. That is, the impeller hub 41 is fixed tothe rotor portion 30, and is arranged to be capable of rotatingintegrally with the rotor portion 30. In addition, the impeller hub 41includes the hub top plate portion 411, which is arranged to extendperpendicularly to the axial direction, and the hub tubular portion 412,which is tubular and is arranged to extend axially downward from anouter edge of the hub top plate portion 411.

Each of the first and second bosses 413 and 415 is arranged to projectaxially downward from an axially lower surface of the hub top plateportion 411. Each of the first and second bosses 413 and 415 is made ofthe same material as that of the hub top plate portion 411, and isdefined integrally with the hub top plate portion 411. Here, the numberof first bosses 413 is four. The first bosses 413 are inserted into thepositioning holes 334 of the rotor yoke 33. The rotor yoke 33 is thuscircumferentially positioned with respect to the impeller hub 41.

Each second boss 415 is arranged to have an axial dimension smaller thanthat of each first boss 413. An upper surface of the rotor top plateportion 331 of the rotor yoke 33 is arranged to be in contact with anaxially lower surface of each second boss 415. That is, the rotor yoke33 is axially positioned with respect to the impeller hub 41 with theupper surface of the rotor top plate portion 331 being in contact withthe axially lower surface of each second boss 415.

Referring to FIGS. 7 and 9, a plurality of gate marks 45 are defined inan upper surface of the hub top plate portion 411 of the impeller hub41. Each gate mark 45 is a mark defined at an inlet (i.e., a gate)defined in a mold (not shown) and through which a resin is injected intothe mold when a resin injection molding process is performed for theimpeller hub 41. The number of gate marks 45 is four, and the four gatemarks 45 are arranged at regular intervals in the circumferentialdirection around the central axis C1.

When the resin is injected into the mold through a plurality of gates, aweld, where different flows of the resin meet, is defined at a middleposition circumferentially between circumferentially adjacent ones ofthe gates. That is, the weld is defined at a middle positioncircumferentially between circumferentially adjacent ones of the gatemarks 45. The weld will be described in detail below.

The blades 42 are arranged side by side in the circumferential directionon an outer surface of the impeller hub 41. In the present preferredembodiment, on the outer surface of the impeller hub 41, the blades 42are arranged side by side at predetermined intervals in thecircumferential direction, and are integrally molded with the impellerhub 41. An upper portion of each blade 42 is arranged forward of a lowerportion of the blade 42 with respect to a rotation direction Rd of theimpeller 40 (see FIG. 2). The upper portion of each blade 42 is arrangedforward of the lower portion of the blade 42 with respect to therotation direction Rd. That is, the impeller hub 41 is provided with theplurality of blades 42, which are arranged in the circumferentialdirection on an outer surface of the hub tubular portion 412.

The blades 42 will now be described in more detail below with additionalreference to FIG. 11. FIG. 11 is a plan view illustrating acircumferential development of one of the blades 42 attached to theimpeller hub 41.

Referring to FIG. 11, a radially innermost portion and a radiallyoutermost portion of the blade 42 will be referred to as an innermostportion 4201 and an outermost portion 4202, respectively. As illustratedin FIG. 11, the innermost portion 4201 is distant from a center of theouter surface of the impeller hub 41 by a distance equal to a radius ofthe outer surface of the impeller hub 41. A first intermediate portion4203 and a second intermediate portion 4204 are defined radially betweenthe innermost portion 4201 and the outermost portion 4202 of the blade42. The innermost portion 4201, the first intermediate portion 4203, thesecond intermediate portion 4204, and the outermost portion 4202 areequally spaced from one another. In other words, the first intermediateportion 4203 corresponds to a radially inner one of two lines thatdivide the blade 42 into three parts having the same radial width. Inaddition, the second intermediate portion 4204 corresponds to a radiallyouter one of the two lines that divide the blade 42 into three partshaving the same radial width.

Referring to FIG. 11, the blade 42 is joined to the impeller hub 41 atthe innermost portion 4201. Meanwhile, radially outside of the innermostportion 4201 of the blade 42, a forward portion of the blade 42 withrespect to the rotation direction Rd includes a portion lying forward ofa foremost portion of the innermost portion 4201 with respect to therotation direction Rd. This portion is not joined to the impeller hub 41in a radial direction, and is therefore low in strength. Accordingly,the forward portion of the blade 42 with respect to the rotationdirection Rd is prone to being deformed radially outward during therotation of the impeller 40. In addition, a rearward portion of theblade 42 with respect to the rotation direction Rd has a reduced radialdimension in a section taken along a plane including the central axisC1, resulting in a reduced section modulus. Accordingly, the rearwardportion of the blade 42 with respect to the rotation direction Rd isalso prone to being deformed radially outward during the rotation of theimpeller 40. Moreover, the rearward portion of the blade 42 with respectto the rotation direction Rd is a portion where an air flow caused bythe rotation of the impeller 40 separates from the blade 42, andtherefore receives an increased stress. This makes the rearward portionof the blade 42 with respect to the rotation direction Rd more prone tobeing deformed radially outward.

While the blade 42 is rotating, a section of the blade 42 taken along aplane including the central axis C1 has a greater section modulus as theradial dimension of the section increases. Thus, a portion of the blade42 which is fixed to the impeller hub 41 in a radial direction is noteasily deformed radially. This characteristic is taken into account todetermine the shape of the blade 42.

A method for determining a portion of the blade 42 which has a largeradial dimension will now be described below with reference to theaccompanying drawings. FIG. 12 is a diagram depicting circumferentialdeveloped blades superimposed on each other. The circumferentialdeveloped blades are circumferential developments of circumferentialsections of the blade 42 taken at different radial positions.

FIG. 12 is a diagram depicting circumferential developments of theinnermost portion 4201, the outermost portion 4202, the firstintermediate portion 4203, and the second intermediate portion 4204 ofthe blade 42. In each of the developments of FIGS. 11 and 12, anupstream end portion of the innermost portion 4201 with respect to therotation direction Rd of the impeller 40 is used as a reference.Referring to FIG. 12, an inner circumferential developed blade 421 is acircumferential development of the innermost portion 4201 of the blade42. Similarly, an outer circumferential developed blade 422 is acircumferential development of the outermost portion 4202 of the blade42, and a first intermediate circumferential developed blade 423 and asecond intermediate circumferential developed blade 424 arecircumferential developments of the first intermediate portion 4203 andthe second intermediate portion 4204, respectively, of the blade 42.

The blade 42 is joined to the impeller hub 41 on a rearward side, withrespect to the rotation direction Rd, of a foremost portion of the innercircumferential developed blade 421 with respect to the rotationdirection Rd. A portion of the blade 42 where all of the innercircumferential developed blade 421, the outer circumferential developedblade 422, the first intermediate circumferential developed blade 423,and the second intermediate circumferential developed blade 424 overlapwhen viewed in the radial direction has a large radial dimension, and istherefore not easily deformed. It is assumed here that the portion ofthe blade 42 where all of the inner circumferential developed blade 421,the outer circumferential developed blade 422, the first intermediatecircumferential developed blade 423, and the second intermediatecircumferential developed blade 424 overlap when viewed in the radialdirection is referred to as a first portion 425. After being determinedwith the circumferential developments superimposed on each other, thefirst portion 425 is transformed from the development back into athree-dimensional space to determine the first portion 425 of the blade42 (see FIGS. 2, 11, and so on). In each of FIGS. 2 and 11, both ends ofthe first portion 425 with respect to the rotation direction areindicated by broken lines.

In the blade 42, the first portion 425 is not easily deformed radiallyoutward during the rotation of the blade 42. Referring to FIG. 2, whenthe impeller 40 has been housed in the air channel wall portion 11 ofthe housing 10, a gap Gp1 between the inner surface of the air channelwall portion 11 and a portion (hereinafter referred to as a “radiallyoutermost portion”) of the first portion 425 of the blade 42 which liesmost radially outward is smaller than a gap Gp2 between the innersurface of the air channel wall portion 11 and a radially outermostportion of a portion of the blade 42 on a forward side of the firstportion 425 with respect to the rotation direction. In addition, the gapGp1 is smaller than a gap Gp3 between the inner surface of the airchannel wall portion 11 and a radially outermost portion of a portion ofthe blade 42 on the rearward side of the first portion 425 with respectto the rotation direction. That is, the radially outermost portion ofthe blade 42 is at the shortest distance from the inner surface of theair channel wall portion 11 in at least a portion of the first portion425.

The distance between the radially outermost portion of the blade 42 andthe inner surface of the air channel wall portion 11 is arranged togradually increase in a forward direction with respect to the rotationdirection from the first portion 425. Similarly, the distance betweenthe radially outermost portion of the blade 42 and the inner surface ofthe air channel wall portion 11 is arranged to gradually increase in arearward direction with respect to the rotation direction from the firstportion 425.

The above arrangement contributes to optimizing a gap between the innersurface of the air channel wall portion 11 and a radially outer edge ofeach blade 42 of the impeller 40 for the rotation of the impeller 40,and increasing efficiency in air blowing by the rotation of the impeller40. The rearward portion of the blade 42 with respect to the rotationdirection is a portion where an air flow separates from the blade 42,and therefore receives greater stress than other portions of the blade42. Accordingly, it is preferable that the radial distance between theradially outermost portion of the blade 42 and the inner surface of theair channel wall portion 11 is greatest at a rearward end portion of theradially outermost portion of the blade 42 with respect to the rotationdirection.

Referring to FIG. 2, when the blade 42 is viewed in the axial direction,the area of the first portion 425 is smaller than a sum of the area of aportion 426 of the blade 42 on the forward side of the first portion 425with respect to the rotation direction and the area of a portion 427 ofthe blade 42 on the rearward side of the first portion 425 with respectto the rotation direction. The above arrangement allows the gap betweenthe radially outermost portion of the blade 42 and the inner surface ofthe air channel wall portion 11 to be adjusted to achieve furtheroptimization of the gap.

Although, in the present preferred embodiment, two portions (i.e., thefirst intermediate portion 4203 and the second intermediate portion4204) of the blade 42 are adopted as radially intermediate portions ofthe blade 42, this is not essential to the present invention. Only oneportion or more than two portions of the blade 42 may alternatively beadopted as the radially intermediate portion(s) of the blade 42. Inaddition, it is preferable that the portion of the first portion 425where the radially outermost portion of the blade 42 is at the shortestradial distance from the inner surface of the air channel wall portion11 is arranged to have a circumferential dimension greater than a halfof the circumferential dimension of the outer circumferential developedblade 422.

The inner fixing portion 43 will now be described in detail below withadditional reference to FIG. 13. FIG. 13 is a schematic bottom viewillustrating an arrangement of the inner fixing portion 43. Referring toFIGS. 8, 10, and 13, the inner fixing portion 43 is arranged radiallyinside of the hub tubular portion 412. The inner fixing portion 43includes wall portions 430 arranged in the circumferential direction.That is, the impeller hub 41 includes the wall portions 430 arranged inthe circumferential direction radially inside of the hub tubular portion412, with a gap between every adjacent ones of the wall portions 430.Each wall portion 430 is arranged to extend axially downward from thelower surface of the hub top plate portion 411. Each wall portion 430 ismolded integrally with the hub top plate portion 411. The inner fixingportion 43 may be tubular, extending in the axial direction.

The wall portions 430 include four first wall portions 431 and foursecond wall portions 432. Each first wall portion 431 includes anincreased thickness portion 433 having a radially inner surface arrangedat a shorter distance from the central axis C1 than any other portion ofa radially inner surface of the first wall portion 431. That is, thefirst wall portion 431 includes the increased thickness portion 433,which has a surface arranged at a shorter distance from the central axisC1 than any other portion of the first wall portion 431. This makes itpossible to press fit the rotor tubular portion 332 to the impeller hub41 at specific circumferential positions. Thus, the rotor tubularportion 332 can be press fitted to, for example, portions of theimpeller hub 41 which are high in strength.

The four first wall portions 431 are arranged at regular intervals inthe circumferential direction. The rotor tubular portion 332 is pressfitted to the inner fixing portion 43 while being in contact with theinner fixing portion 43, more specifically, with some of the wallportions 430. Referring to FIG. 13, the radially inner surface of theincreased thickness portion 433 is arranged to be in contact with anouter surface of the rotor tubular portion 332. The rotor tubularportion 332 is press fitted to the inner fixing portion 43 while beingin contact with the increased thickness portion 433. The distance of theincreased thickness portion 433 from the central axis C1 is arranged tocontinuously increase from a circumferential middle of the increasedthickness portion 433 in circumferentially outward directions. That is,the distance of the increased thickness portion 433 from the centralaxis C1 continuously increases from the circumferential middle of theincreased thickness portion 433 in the circumferentially outwarddirections. Thus, an improvement in strength of the increased thicknessportion 433 is achieved. In addition, molding of the increased thicknessportion 433 is made easier.

That is, the outer surface of the rotor tubular portion 332 is arrangedto be in contact with an inner surface of at least one of the wallportions 430. Thus, when the rotor tubular portion 332 is press fittedto the impeller hub 41, the rotor tubular portion 332 is brought intocontact with only some of the wall portions 430, and therefore, controlof press-fitting strength is easy. In addition, a reduction in thelikelihood that an occurrence of thermal expansion caused by atemperature change, for example, will cause an excessively high orexcessively low press-fitting strength is achieved.

Each second wall portion 432 is arranged circumferentially betweenadjacent ones of the first wall portions 431. The four second wallportions 432 are arranged at regular intervals in the circumferentialdirection. That is, the first wall portions 431 and the second wallportions 432 are arranged alternately and at regular intervals in thecircumferential direction.

A weld 434 is defined in a circumferential middle of a radially innersurface of each second wall portion 432. The weld 434 is a portion ofthe second wall portion 432 where flows of the resin coming fromdifferent directions have met, and is therefore lower in strength thanother portions of the second wall portion 432. Accordingly, when therotor tubular portion 332 is press fitted to the inner fixing portion43, more specifically, to some of the wall portions 430, the radiallyinner surface of the second wall portion 432 is arranged opposite to theouter surface of the rotor tubular portion 332 with a gap therebetweento prevent a concentration of stress on the weld 434 at the time of thepress fitting or during the rotation of the impeller 40.

The inner fixing portion 43 includes first regions 4301 each of which isarranged radially opposite to the outer surface of the rotor tubularportion 332 with a gap therebetween, and each of which has the weld 434defined in a radially inner surface thereof. The inner fixing portion 43also includes second regions 4302 each of which is arranged to be incontact with the outer surface of the rotor tubular portion 332.

Each weld 434 is defined at a middle position between adjacent ones ofthe gate marks 45. Each second region 4302 is arranged circumferentiallybetween adjacent ones of the first regions 4301. Accordingly, eachsecond region 4302 is arranged in a region between circumferentiallyadjacent ones of the welds 434. That is, at least a portion of the outersurface of the rotor tubular portion 332 is arranged to be in contactwith an inner surface of a portion of the inner fixing portion 43 (i.e.,the second region 4302) which lies in a region between a middle positionbetween each gate mark 45 and a circumferentially adjacent one of thegate marks 45 and a middle position between the gate mark 45 and anothercircumferentially adjacent one of the gate marks 45. Each first wallportion 431 is arranged on an imaginary line VL that joins acorresponding one of the gate marks 45 and the central axis C1 (seeFIGS. 3, 13, and so on).

Referring to FIGS. 3, 8, and 10, the impeller hub 41 includes a recessedportion 46 being recessed axially upward from an axially lower endportion thereof radially between the hub tubular portion 412 and thewall portions 430. Each wall portion 430 is joined to a radially innersurface of the hub tubular portion 412 through a joining portion(s) 44arranged in the recessed portion 46. That is, the impeller hub 41includes the joining portions 44, each of which is arranged to join acorresponding one of the wall portions 430 to the hub tubular portion412. In addition, each of the first wall portions 431 (i.e., the atleast one of the wall portions 430 arranged to be in contact with theouter surface of the rotor tubular portion 332) is joined to the hubtubular portion 412 through two or more of the joining portions 44arranged in the circumferential direction. This allows press-fittingstress to be distributed, and contributes to preventing a deformation ofthe impeller hub 41. Each joining portion 44 is arranged to extend inthe axial direction. The recessed portion 46 is arranged to have anaxial dimension smaller than that of the impeller hub 41. That is, theimpeller hub 41 includes the recessed portion 46 being recessed upwardand defined radially between the hub tubular portion 412 and the wallportions 430, and the recessed portion 46 is arranged to have an axialdepth smaller than the axial dimension of the impeller hub 41. Thisleads to increased strength of a region near an upper end portion ofeach wall portion 430. In addition, a mold needs to be inserted only ashort distance, making the molding easier.

Referring to FIG. 10, both circumferential ends of each first wallportion 431 are joined to the hub tubular portion 412 through thecorresponding joining portions 44. A circumferential middle portion ofthe first wall portion 431 defines the increased thickness portion 433.The circumferential middle portion of the first wall portion 431 and thehub tubular portion 412 are arranged radially opposite to each otherwith the recessed portion 46 therebetween. The above arrangement allowsthe first wall portion 431 to bend. This allows stress to be distributedwhen the rotor tubular portion 332 is press fitted to the inner fixingportion 43. The first wall portion 431 is made of the resin, while therotor tubular portion 332 is made of the metal. Accordingly, an increasein temperature of the first wall portion 431 and the rotor tubularportion 332 will cause the first wall portion 431 to experience agreater thermal expansion than the rotor tubular portion 332. Provisionof the recessed portion 46 between the hub tubular portion 412 and aradially outer side of the circumferential middle portion of the firstwall portion 431 allows the first wall portion 431 to be deformedradially outward. This contributes to limiting an increase in stresscaused by a difference in thermal expansion at an area of contactbetween the first wall portion 431 and the rotor tubular portion 332.

That is, both circumferential end portions of each of the first wallportions 431 are joined to the hub tubular portion 412 through thecorresponding joining portions 44. This leads to increased strength ofboth the circumferential end portions of each first wall portion 431. Inaddition, a remaining portion of each first wall portion 431, excludingboth the circumferential end portions thereof, is increased inelasticity, which contributes to a distribution of stress at the time ofthe press fitting. Further, each of the first wall portions 431 isarranged to be in contact with the outer surface of the rotor tubularportion 332 at a circumferential middle portion thereof. Thus, the rotortubular portion 332 can be press fitted to a portion of each first wallportion 431 which has a relatively high elasticity, which contributes toa distribution of the stress at the time of the press fitting. Theremaining portion of each of the first wall portions 431, excluding boththe circumferential end portions thereof, is arranged radially oppositeto the hub tubular portion 412 with a gap therebetween. This contributesto a distribution of stress caused by the press fitting, and topreventing a deformation and a distortion of the impeller hub 41. Thisin turn reduces the likelihood of a distortion or a bending of any blade42, and the likelihood of a reduction in air blowing efficiency.

A radially outer side of a circumferential middle portion of each secondwall portion 432 is joined to the radially inner surface of the hubtubular portion 412 through the corresponding joining portion 44. Eachsecond wall portion 432 is joined to the radially inner surface of thehub tubular portion 412 through a single one of the joining portions 44.As mentioned above, the circumferential middle portion of the secondwall portion 432 includes the weld 434. A portion of the second wallportion 432 in which the weld 434 is defined is joined to the radiallyinner surface of the hub tubular portion 412 through the correspondingjoining portion 44, so that an increase in strength of the portion ofthe second wall portion 432 in which the weld 434 is defined can beachieved. In addition, a difference in thermal expansion between thesecond wall portion 432 and the rotor tubular portion 332 would not leadto a significant increase in stress, since the second wall portion 432and the rotor tubular portion 332 are radially spaced apart from eachother with the gap therebetween.

The above arrangement contributes to preventing an increase in stress onthe inner fixing portion 43 (i.e., the wall portions 430) even if thetemperatures of the inner fixing portion 43 (i.e., the wall portions430) and the rotor tubular portion 332 become higher when the axial fanA is running than when the rotor yoke 33 was press fitted to theimpeller hub 41. This in turn contributes to reducing a change ininternal stress caused by a temperature change between the time ofmanufacture (i.e., the time of the press fitting) and the time when theaxial fan A is running, and thus allows the impeller 40 to rotate withstability.

An axial fan according to a second preferred embodiment of the presentdisclosure will now be described below with reference to theaccompanying drawings. FIG. 14 is a vertical sectional view of animpeller 40B used in the axial fan according to the second preferredembodiment of the present disclosure. FIG. 15 is a schematic bottom viewof one of first wall portions 471 included in the impeller 40Billustrated in FIG. 14. The axial fan according to the second preferredembodiment is similar in structure to the axial fan A according to thefirst preferred embodiment except in the structure of the impeller 40B.Accordingly, detailed descriptions of members of the axial fan accordingto the second preferred embodiment other than the impeller 40B areomitted.

Referring to FIG. 14, the impeller 40B is different in structure fromthe impeller 40 according to the first preferred embodiment in that wallportions 47 are provided in place of the wall portions 430 of theimpeller 40. The wall portions 47 include the first wall portions 471and second wall portions 472. Each second wall portion 472 issubstantially identical in structure to each second wall portion 432 ofthe impeller 40. That is, each second wall portion 472 is arrangedradially opposite to a rotor tubular portion 332 with a gaptherebetween.

Referring to FIGS. 14 and 15, each first wall portion 471 includes a rib473 arranged to project radially inward in a circumferential middle of aradially inner surface thereof. In the first wall portion 471, the rib473 defines an increased thickness portion. That is, the increasedthickness portion is defined by the rib 473, which is arranged toproject radially inward from the inner surface of the first wall portion471. Accordingly, when a rotor yoke 33 has been press fitted to animpeller hub 41 of the impeller 40B, a radially outer surface of therotor tubular portion 332 is in contact with a radially inner surface ofthe rib 473. This contributes to preventing a concentration of stress atthe time of the press fitting on an outer edge of a hub top plateportion 411, which in turn contributes to preventing a deformation ofthe impeller hub 41.

Referring to FIG. 14, a gap is defined between the rib 473 and anaxially lower surface of the hub top plate portion 411. The hub topplate portion 411 includes, in a region axially opposed to the rib 473,a through hole 48 arranged to pass therethrough in the axial direction.That is, the increased thickness portion 473 is arranged axiallyopposite to the hub top plate portion 411 with the gap therebetween.This contributes to preventing a concentration of stress at the time ofthe press fitting on the outer edge of the hub top plate portion 411when a rotor portion 30 is press fitted to the impeller hub 41. This inturn contributes to preventing a deformation of the impeller hub 41. Inaddition, the hub top plate portion 411 includes, in the region axiallyopposed to the increased thickness portion 473, the through hole 48arranged to pass therethrough in the axial direction. This allows theimpeller hub 41, which includes the first wall portions 471 each ofwhich includes the increased thickness portion 473, to be molded using amold which is to be drawn in the axial direction, i.e., the verticaldirection, which leads to a reduction in production cost.

The gap defined between an upper end of the rib 473 and the hub topplate portion 411 reduces the likelihood that a press-fitting stresswill be transferred to a hub tubular portion 412 of the impeller hub 41when the rotor yoke 33 is press fitted to the impeller hub 41. Thiscontributes to preventing a deformation of the impeller hub 41.

Provision of the through hole 48 allows the gap between the rib 473 andthe hub top plate portion 411 to be defined using an insert (i.e., amold) which is to be drawn in the axial direction in a resin injectionmolding process. This allows use of a mold having a simplifiedstructure.

The second preferred embodiment is otherwise similar to the firstpreferred embodiment.

An axial fan according to a third preferred embodiment of the presentdisclosure will now be described below with reference to theaccompanying drawings. FIG. 16 is a bottom view of an impeller 40C usedin the axial fan according to the third preferred embodiment of thepresent disclosure. The axial fan according to the third preferredembodiment is similar in structure to the axial fan A according to thefirst preferred embodiment except in the structure of the impeller 40C.Accordingly, detailed descriptions of members of the axial fan accordingto the third preferred embodiment other than the impeller 40C areomitted.

Referring to FIG. 16, the impeller 40C is different in structure fromthe impeller 40 in that an inner fixing portion 49 is provided in placeof the inner fixing portion 43 of the impeller 40. The inner fixingportion 49 includes first projecting portions 491 and second projectingportions 492. Each of the first and second projecting portions 491 and492 is arranged to project radially inward from a radially inner surfaceof a hub tubular portion 412. Each second projecting portion 492 isarranged to project radially inward to a greater extent than each firstprojecting portion 491. Accordingly, when a rotor yoke 33 has been pressfitted to an impeller hub 41, a radially outer surface of a rotortubular portion 332 is in contact with a radially inner surface of eachsecond projecting portion 492.

Referring to FIG. 16, a weld 494 is defined in a radially inner surfaceof each first projecting portion 491. Each second projecting portion 492is arranged radially outside of a corresponding one of gate marks 45. Inmore detail, each second projecting portion 492 is arranged on animaginary line VL that joins the corresponding gate mark 45 and acentral axis C1.

With the above arrangement, the weld 494 is defined in each firstprojecting portion 491, on which a stress at the time of the pressfitting will not act. In addition, each second projecting portion 492,which is arranged to be in contact with the rotor tubular portion 332,is arranged in the vicinity of the corresponding gate mark 45, where ahigh strength is provided. This contributes to preventing a deformationwhen the rotor yoke 33 is press fitted to the impeller hub 41. Eachfirst projecting portion 491 includes a first region 4901 arrangedopposite to the radially outer surface of the rotor tubular portion 332with a gap therebetween. Each second projecting portion 492 includes asecond region 4902 arranged to be in radial contact with the radiallyouter surface of the rotor tubular portion 332.

The third preferred embodiment is otherwise similar to the firstpreferred embodiment.

An axial fan according to a preferred embodiment of the presentdisclosure may be used in, for example, a blower apparatus or the like.The blower apparatus may be used, for example, to cool an electronicdevice.

Features of the above-described preferred embodiments and themodifications thereof may be combined appropriately as long as noconflict arises.

While preferred embodiments of the present disclosure have beendescribed above, it is to be understood that variations andmodifications will be apparent to those skilled in the art withoutdeparting from the scope and spirit of the present disclosure. The scopeof the present disclosure, therefore, is to be determined solely by thefollowing claims.

What is claimed is:
 1. An axial fan comprising: a rotor portionincluding a shaft arranged to extend along a central axis extending in avertical direction; a stator portion arranged radially opposite to therotor portion; and an impeller hub fixed to the rotor portion, andarranged to be capable of rotating integrally with the rotor portion;wherein the impeller hub includes: a hub top plate portion arranged toextend perpendicularly to an axial direction; a hub tubular portionbeing tubular, and arranged to extend axially downward from an outeredge of the hub top plate portion; a plurality of blades arranged in acircumferential direction on an outer surface of the hub tubularportion; a plurality of wall portions arranged in the circumferentialdirection radially inside of the hub tubular portion, with a gap betweenevery adjacent ones of the wall portions; and a plurality of joiningportions each of which is arranged to join a corresponding one of thewall portions to the hub tubular portion; the rotor portion includes arotor tubular portion being tubular and arranged to extend in the axialdirection; an outer surface of the rotor tubular portion is arranged tobe in contact with an inner surface of at least one of the wallportions; and each of the at least one wall portion arranged to be incontact with the outer surface of the rotor tubular portion is joined tothe hub tubular portion through two or more of the joining portionsarranged in the circumferential direction.
 2. The axial fan according toclaim 1, wherein both circumferential end portions of each of the atleast one wall portion arranged to be in contact with the outer surfaceof the rotor tubular portion are joined to the hub tubular portionthrough the corresponding joining portions.
 3. The axial fan accordingto claim 1, wherein each of the at least one wall portion arranged to bein contact with the outer surface of the rotor tubular portion isarranged to be in contact with the outer surface of the rotor tubularportion at a circumferential middle portion thereof.
 4. The axial fanaccording to claim 3, wherein a remaining portion of each of the atleast one wall portion arranged to be in contact with the outer surfaceof the rotor tubular portion, excluding both circumferential endportions thereof, is arranged radially opposite to the hub tubularportion with a gap therebetween.
 5. The axial fan according to claim 1,wherein each of the at least one wall portion arranged to be in contactwith the outer surface of the rotor tubular portion includes anincreased thickness portion having a surface arranged at a shorterdistance from the central axis than any other portion of the wallportion.
 6. The axial fan according to claim 5, wherein a distance ofthe increased thickness portion from the central axis is arranged tocontinuously increase from a circumferential middle of the increasedthickness portion in circumferentially outward directions.
 7. The axialfan according to claim 5, wherein the increased thickness portion isdefined by a rib arranged to project radially inward from an innersurface of the wall portion.
 8. The axial fan according to claim 5,wherein the increased thickness portion is arranged axially opposite tothe hub top plate portion with a gap therebetween.
 9. The axial fanaccording to claim 8, wherein the hub top plate portion includes, in aregion axially opposed to the increased thickness portion, a throughhole arranged to pass therethrough in the axial direction.
 10. The axialfan according to claim 1, wherein the impeller hub further includes arecessed portion being recessed upward and defined radially between thehub tubular portion and the wall portions; and the recessed portion isarranged to have an axial depth smaller than an axial dimension of theimpeller hub.